1. Field of the Invention
The present invention relates to optical communication equipment and, more specifically, to equipment for processing optical duobinary signals.
2. Description of the Related Art
Duobinary signaling was introduced in the 1960s and since then has found numerous applications in communication systems. The principle of duobinary signaling is explained, for example, in an article by A. Lender that appeared in IEEE Transactions on Communications and Electronics, Vol. 82 (May, 1963), pp. 214-218, the teachings of which are incorporated herein by reference. Briefly, duobinary signaling uses three signal levels, for example, “+1”, “0”, and “−1”. A signal corresponding to one of these levels (i.e., a duobinary symbol) is transmitted during each signaling interval (time slot). A duobinary signal is typically generated from a corresponding binary signal using certain transformation rules. Although both signals carry the same information, the bandwidth of the duobinary signal may be reduced by a factor of 2 compared to that of the binary signal. In addition, the duobinary signal may be constructed such that it has certain inter-symbol correlation (ISC) data, which can be used to implement an error-correction algorithm at the receiver.
A number of different transformations have been proposed for constructing a duobinary sequence, bk, from a corresponding binary sequence, ak, where k=1, 2, 3, . . . One such transformation described in the above-cited Lender article is as follows. For any particular k=m, when am=0, bm=0. When am=1, bm equals either +1 or −1, with the polarity of bm determined based on the polarity of last non-zero symbol bm-i preceding bm, where i is a positive integer. More specifically, when i is odd, the polarity of bm is the same as the polarity of bm-i; and, when i is even, the polarity of bm is the opposite of the polarity of bm-i. Due to the properties of this transformation, the duobinary sequence has no transitions between the “+1” and “−1” levels in successive time slots. Only transitions between (i) “0” and “+1” and (ii) “0” and “−1” levels can occur. Reconstruction of ak from a known bk is relatively straightforward. More specifically, when bm=±1, am=1; and, when bm=0, am=0.
In optical communication systems, duobinary encoding is typically implemented using phase modulation of a carrier optical beam disclosed in U.S. Pat. No. 5,867,534, the teachings of which are incorporated herein by reference. More specifically, for the “0” bit, substantially no light is transmitted. However, the “+1” and “−1” bits are transmitted as light having +E and −E electric fields, respectively, where opposite polarities of the electric field correspond to a relative optical phase shift of 180 degrees. While an optical beam modulated in this manner is a three-level signal in terms of the electric field, it is a two-level signal in terms of the optical power. Based on this property of duobinary signals, a “binary” receiver may be adapted to serve as a duobinary receiver. A conventional binary receiver simply measures optical power. Since both “+1” and “−1” duobinary states correspond to light “on”, a binary receiver can convert optical duobinary input signals into electrical output signals by measuring optical power. However, it would be desirable to have a specialized duobinary receiver, which, when deployed in a communication system in place of a regular binary receiver, would improve the system performance using advantages of optical duobinary coding.